Dry buttermilk and regular nonfat dry milk, which are sold in bulk to producers of ice cream, bakery products, and some processed meat processors, can be graded either U.S. Extra or U.S. Standard. The lower grade, "Standard," may be the result of excess moisture or scorched particles from the drying process or certain other quality factors. U.S. Standard Grade Nonfat Dry Milk when reconstituted will be fairly pleasing in flavor, but may have the following flavors to a slight degree: Bitter, oxidized, scorched, storage, or utensil. Nonfat dry milk may have the following flavors to a definite degree: chalky, cooked, feed, or flat. Nonfat dry milk may have a slightly unnatural color, but should be free from lumps.
http://www.ams.usda.gov